Order-disorder component in the phase transition mechanism of 18O enriched strontium titanate

Precision Multi-Mode Dielectric Characterization of a Crystalline Perovskite Enables Determination of the Temperature-Dependent Phase Transitions

Simple perovskite crystals undergo structural phase transitions on cooling to low temperatures, which significantly change the material properties of the crystal. In this work, we rigorously characterize the temperature evolution of permittivity of a perovskite crystal as it undergoes phase transitions. In particular, we have undertaken precision measurements of a single crystal of Strontium Titanate from 294.6 to 5.6 K, by measuring the frequency of multiple microwave transverse electric (TE) and magnetic resonant modes simultaneously. The multi-mode microwave measurement technique of resonant frequency used in this work allows high precision determination of any induced anisotropy of the permittivity as the crystal undergoes structural phase transitions. Compared with previous results, we unequivocally show that the permittivity has an isotropic value of 316.3±2.2 at room temperature, consistent with its well-known cubic structure, and determine the onset of dielectric anisotropy as the crystal is cooled to lower temperatures. We show that the crystal exhibits uniaxial anisotropy in the permittivity below 105 K when the structure becomes tetragonal, and exhibits biaxial anisotropy in the permittivity below 51 K when the structure becomes orthorhombic.

Non-Ohmic Variable-Range Hopping and Resistive Switching in SrTiO_{3} Domain Walls

We report observation of electric field driven conductivity with negative differential conductance and resistive switching in insulating SrTiO_{3} samples over a wide range of applied voltages at low temperatures. The observed current follows I=I_{0}exp[-(E^{*}/E)^{1/2}] at large applied electric field, corresponding to variable range hopping conduction with a Coulomb gap in domain walls. Our data are sufficient to discriminate unambiguously between Shklovskii and Mott hopping via their different electric field exponent. Under some conditions space-charge-limited currents are observed, and the charge mobility limit is determined to be in the range of 17 and 210  cm^{2}/Vs.

Domain Dynamics in Quantum-Paraelectric SrTiO_{3}

Twin dynamics forced by acoustic waves shows several linear and nonlinear response modes below T_{c}=106  K. In the quantum paraelectric state a "quantum domain glass" at 25  K<T<40  K shows intense relaxation and temperature hysteresis. Domains float collectively in a complex, smooth landscape with long relaxation times. In the "quantum domain solid" state below 25 K new phenomena occur. A temperature-dependent memory effect of the elastic response after anneal at 36 K depends on the lowest temperature reached in the quantum domain solid state below 25 K. The glassiness of twin boundary dynamics vanishes for temperatures approaching absolute zero.

Magnetic Domain Walls and Macroscopic Magnetization-Related Elastic and Anelastic Effects during Premartensitic Transition in Ni₂MnGa

The temperature and field dependences of internal friction and Young´s modulus are studied using a high-resolution ultrasonic (90 kHz) technique in stoichiometric ferromagnetic Ni₂MnGa shape memory alloy close to the premartensitic transformation temperature, T_PM, in the demagnetized state and under moderate fields. Several new effects observed like an apparent Young´s modulus softening close to T_PM under moderate fields, instead of the hardening outside this range, as well as existing controversies in the apparent elastic and anelastic properties of Ni₂MnGa close to T_PM are explained by microeddy and macroeddy current relaxations that to date have been disregarded.

Glasslike Dynamics of Polar Domain Walls in Cryogenic SrTiO_{3}

Polar and highly mobile domain walls in SrTiO_{3} move under electric and elastic fields. Two vastly different timescales dominate their dynamical behavior. The previously observed fast changes lead to anomalies near 40 K where the elastic moduli soften and the polarity of the walls becomes strong. Keeping the sample under isothermal conditions leads to a new and unexpected phenomenon: The softening vanishes over timescales of days while the piezoelectricity of the sample remains unchanged. The hardening follows glass dynamics below an onset at T^{*}≈40  K. The timescale of the hardening is strongly temperature dependent and can be followed experimentally down to 34 K when the relaxation is not completed within two days. The relaxation time of a stretched exponential decay increases exponentially with the decreasing temperature. This relaxation process follows similar dynamics after zero-field cooling and after applying or removing an electric field. The sluggish behavior is attributed to collective interactions of domain patterns following overdamped glass dynamics rather than ballistic dynamics.

Imaging and tuning polarity at SrTiO3 domain walls

Electrostatic fields tune the ground state of interfaces between complex oxide materials. Electronic properties, such as conductivity and superconductivity, can be tuned and then used to create and control circuit elements and gate-defined devices. Here we show that naturally occurring twin boundaries, with properties that are different from their surrounding bulk, can tune the LaAlO₃/SrTiO₃ interface 2DEG at the nanoscale. In particular, SrTiO₃ domain boundaries have the unusual distinction of remaining highly mobile down to low temperatures, and were recently suggested to be polar. Here we apply localized pressure to an individual SrTiO₃ twin boundary and detect a change in LaAlO₃/SrTiO₃ interface current distribution. Our data directly confirm the existence of polarity at the twin boundaries, and demonstrate that they can serve as effective tunable gates. As the location of SrTiO₃ domain walls can be controlled using external field stimuli, our findings suggest a novel approach to manipulate SrTiO₃-based devices on the nanoscale.

Superconductivity-induced magnetization depletion in a ferromagnet through an insulator in a ferromagnet-insulator-superconductor hybrid oxide heterostructure

Coupling between superconducting and ferromagnetic states in hybrid oxide heterostructures is presently a topic of intense research. Such a coupling is due to the leakage of the Cooper pairs into the ferromagnet. However, tunneling of the Cooper pairs though an insulator was never considered plausible. Using depth sensitive polarized neutron reflectivity we demonstrate the coupling between superconductor and magnetic layers in epitaxial La2/3Ca1/3MnO3 (LCMO)/SrTiO3/YBa2Cu3O7-δ (YBCO) hybrid heterostructures, with SrTiO3 as an intervening oxide insulator layer between the ferromagnet and the superconductor. Measurements above and below the superconducting transition temperature (TSC) of YBCO demonstrate a large modulation of magnetization in the ferromagnetic layer below the TSC of YBCO in these heterostructures. This work highlights a unique tunneling phenomenon between the epitaxial layers of an oxide superconductor (YBCO) and a magnetic layer (LCMO) through an insulating layer. Our work would inspire further investigations on the fundamental aspect of a long range order of the triplet spin-pairing in hybrid structures.

Polar domain walls trigger magnetoelectric coupling

Interface physics in oxides heterostructures is pivotal in material’s science. Domain walls (DWs) in ferroic systems are examples of naturally occurring interfaces, where order parameter of neighboring domains is modified and emerging properties may develop. Here we show that electric tuning of ferroelastic domain walls in SrTiO₃ leads to dramatic changes of the magnetic domain structure of a neighboring magnetic layer (La_1/2Sr_1/2MnO₃) epitaxially clamped on a SrTiO₃ substrate. We show that the properties of the magnetic layer are intimately connected to the existence of polar regions at twin boundaries of SrTiO₃, developing at , that can be electrically modulated. These findings illustrate that by exploiting the responsiveness of DWs nanoregions to external stimuli, even in absence of any domain contribution, prominent and adjustable macroscopic reactions of neighboring layers can be obtained. We conclude that polar DWs, known to exist in other materials, can be used to trigger tunable responses and may lead to new ways for the manipulation of interfacial emerging properties.

Domains within domains and walls within walls: evidence for polar domains in cryogenic SrTiO3

Resonant piezoelectric spectroscopy shows polar resonances in paraelectric SrTiO3 at temperatures below 80 K. These resonances become strong at T<40  K. The resonances are induced by weak electric fields and lead to standing mechanical waves in the sample. This piezoelectric response does not exist in paraelastic SrTiO3 nor at temperatures just below the ferroelastic phase transition. The interpretation of the resonances is related to ferroelastic twin walls which become polar at low temperatures in close analogy with the known behavior of CaTiO3. SrTiO3 is different from CaTiO3, however, because the wall polarity is thermally induced; i.e., there exists a small temperature range well below the ferroelastic transition point at 105 K where polarity appears on cooling. As the walls are atomistically thin, this transition has the hallmarks of a two-dimensional phase transition restrained to the twin boundaries rather than a classic bulk phase transition.

Electric-field-induced polar order and localization of the confined electrons in LaAlO3/SrTiO3 heterostructures

With ellipsometry, x-ray diffraction, and resistance measurements we investigated the electric-field effect on the confined electrons at the LaAlO3/SrTiO3 interface. We obtained evidence that the localization of the electrons at negative gate voltage is induced, or at least enhanced, by a polar phase transition in SrTiO3 which strongly reduces the lattice polarizability and the subsequent screening. In particular, we show that the charge localization and the polar order of SrTiO3 both develop below ∼50  K and exhibit similar, unipolar hysteresis loops as a function of the gate voltage.

Domain wall damping and elastic softening in SrTiO3: evidence for polar twin walls

A marked change in anelastic properties, namely, elastic softening accompanied by increased damping, has been observed in a single crystal of SrTiO(3) below ~50 K by resonant ultrasound spectroscopy. This correlates with other subtle changes in structure and properties which have been explained in the past in terms of a novel quantum state and the formation of polar clusters in an incipient ferroelectric structure. Comparison of the new data, obtained at frequencies near 1 MHz, with mechanical spectroscopy data collected at a few Hz or a few kHz, reveals a distinct dispersion with frequency and is interpreted in terms of an acoustic loss mechanism which depends primarily on the mobility under stress of ferroelastic twin walls. In most ferroelastic materials, it is found that the twin walls become immobile below some low-temperature interval due to the pinning effects of defects. It is proposed instead for SrTiO(3) that associated with the local atomic displacements within the incipient ferroelectric clusters is a change in structure of the twin walls such that their mobility becomes enhanced. We propose that the structural change is not correlated with structural changes of the bulk material but relates to increasing polarity of the walls. This interpretation implies that ferroelastic domain walls in SrTiO(3) become ferroelectric at low temperatures.

The polarizability model for ferroelectricity in perovskite oxides

This article reviews the polarizability model and its applications to ferroelectric perovskite oxides. The motivation for the introduction of the model is discussed and nonlinear oxygen ion polarizability effects and their lattice dynamical implementation outlined. While a large part of this work is dedicated to results obtained within the self-consistent-phonon approximation, nonlinear solutions of the model are also handled, which are of interest to the physics of relaxor ferroelectrics, domain wall motions, and incommensurate phase transitions. The main emphasis is to compare the results of the model with experimental data and to predict novel phenomena.

Pseudo Jahn-Teller origin of perovskite multiferroics, magnetic-ferroelectric crossover, and magnetoelectric effects: the d0-d10 problem

The conditions of multiferroicity in d(n) perovskites are derived from the pseudo Jahn-Teller effect, due to which ferroelectric displacements are triggered by vibronic coupling between ground and excited electronic states of opposite parity but same spin multiplicity; it takes place for some specific d(n) configurations and spin states only. In combination with the high-spin-low-spin crossover effect this leads to a novel phenomenon, the magnetic-ferroelectric (multiferroics) crossover which predicts magnetoelectric effects with exciting functionalities including electric magnetization and demagnetization.

Elastic and anelastic properties of ferroelectric SrTi(18)O3 in the kHz-MHz regime

Resonant ultrasound spectroscopy has been used to follow elastic softening in SrTi(18)O3 in the frequency range ∼0.2-1  MHz. A dramatic softening of C44 occurs as the Curie temperature T(c) = 24  K is approached from above or below, which correlates with the development of a central peak in Raman and Brillouin spectra. This is attributed to strong coupling between the acoustic mode and the central peak mode. A weaker anomaly is seen in a resonance mode which is believed to be controlled by 1/2(C11-C12). Significant attenuation accompanies this softening and an additional dissipation peak has also been observed at ∼80-90  K. This extends earlier work by a factor of 150,000× from the 30 GHz regime and helps address the question as to whether the ferroelectricity is stimulated primarily by a soft mode into a homogeneous ground state or by clustering of rhombohedral nanoregions into an inhomogeneous ground state.

Polar-Soft-Mode-Driven Structural Phase Transition in SrTiO3

The structural phase transition of SrTiO3 at 105 K, which has been believed to be independent of the ferroelectric soft mode [Phys. Rev. 177, 858 (1969)], is shown, on the contrary, to be driven by the same long-wavelength polar instability. Isotope replacement of 16O by 18O is predicted to cause an increase in the structural phase transition temperature by 3.8 K. In both isotopic cases, dynamical polarizability-induced ferroelastic-type cluster formation takes place above the structural phase transition, which is intrinsic and a consequence of electron-lattice driven mode-mode coupling. Distinct length and time scales are identified. The precursor domains are evidence that order-disorder effects coexist with displacive dynamics.

Ideal soft mode-type quantum phase transition and phase coexistence at quantum critical point in 18O-exchanged SrTiO3

The quantum ferroelectric phase transition of 18O-exchanged SrTiO3 (x% exchanged SrTiO3 is abbreviated as STO18-x) was investigated by Raman scattering as a function of x. The result indicates the ideal soft mode-type quantum ferroelectric phase transition of STO18-x, where the 18O exchange enhances the softening of the soft mode by the suppression of quantum fluctuation. In the vicinity of the quantum critical point (x approximately xc=33%), the system results in the ferroelectric-paraelectric phase coexistence state, in clear contrast to the homogeneous ferroelectric phase in STO18-x, whose x is sufficiently larger than xc. Simultaneously, the softening of the soft mode becomes strongly rounded with the underdamped oscillation. The present result indicates that the sensitivity of the soft phonon vibration to the mass disorder is dramatically enhanced in the vicinity of the quantum critical point.